In the past considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are rapidly being depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel vessel or tank of common design only about 1% of the total weight is comprised of hydrogen gas when it is stored in the tank at a typical pressure of 136 atmospheres. In order to obtain equivalent amounts of energy, a container of hydrogen gas weighs about thirty times the weight of a container of gasoline.
Hydrogen also can be stored as a liquid. Storage as a liquid, however, presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below -253 degree C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Storage of hydrogen as a solid hydride can provide a greater percent weight storage than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
A high hydrogen storage capacity per unit weight of material is an important consideration in applications where the hydride does not remain stationary. A low hydrogen storage capacity relative to the weight of the material reduces the mileage and hence the range of the vehicle making the use of such materials impractical. A low desorption temperature is desirable to reduce the amount of energy required to release the hydrogen. Furthermore, a relatively low desorption temperature to release the stored hydrogen is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
The prior art metallic host hydrogen storage materials include magnesium, magnesium nickel, vanadium, iron-titanium, lanthanum pentanickel and alloys of these metals. No prior art material, however, has all of the required properties required for a storage medium with widespread commercial utilization.
Thus, while many metal hydride systems have been proposed, the Mg systems have been heavily studied ever since hydrogen storage was first reported in Mg.sub.2 Ni. The Mg systems are of high interest because of their large hydrogen storage capacity. However, as discussed above, Mg hydrogen storage systems have not been used commercially to date because of the disadvantages inherent therein.
While magnesium can store large amounts of hydrogen, it's primary disadvantages are high absorption-desorption temperature, low plateau pressure, extremely slow kinetics and large heat of hydride formation (.about.75 kJ/mol). For example, magnesium hydride is theoretically capable of storing hydrogen at approximately 7.6% by weight computed using the formula: percent storage=H/H+M, where H is the weight of the hydrogen stored and M is the weight of the material to store the hydrogen (all storage percentages hereinafter referred to are computed based on this formula). While a 7.6% storage capacity is suited for on board hydrogen storage for use in powering vehicles, magnesium's other hydrogen storage characteristics make it commercially unacceptable for widespread use.
Magnesium is very difficult to activate. For example, U.S. Pat. No. 3,479,165 discloses that it is necessary to activate magnesium to eliminate surface barriers at temperatures of 400.degree. C. to 425.degree. C. and 1000 psi for several days to obtain a reasonable (90%) conversion to the hydride state. Furthermore, desorption of such hydrides typically requires heating to relatively high temperatures before hydrogen desorption begins. The aforementioned patent states that the MgH.sub.2 material must be heated to a temperature of 277.degree. C. before desorption initiates, and significantly higher temperatures and times are required to reach an acceptable operating output. The high desorption temperature makes the magnesium hydride unsuitable for many applications, in particular applications wherein it is desired to utilize waste heat for desorption such as the exhaust heat from combustion engines.
Mg-based alloys have been considered for hydrogen storage also. The two main Mg alloy crystal structures investigated have been the A.sub.2 B and AB.sub.2 alloy systems. In the A.sub.2 B system, Mg.sub.2 Ni alloys have been heavily studied because of their moderate hydrogen storage capacity, and lower heat of formation (64 kJ/mol) than Mg. However, because Mg.sub.2 Ni has a storage capacity of only 3.6 wt. % hydrogen, researchers have attempted to improve the hydrogenation properties of these alloys through mechanical alloying, mechanical grinding and elemental substitutions. For instance, in U.S. Pat. Nos. 5,506,069 and 5,616,432 (the disclosures of which are incorporated herein by reference), Ovshinsky et al, have modified non-Laves phase Mg-Ni alloys for electrochemical work.
More recently, investigators have attempted to form MgNi.sub.2 type alloys for use in hydrogen storage. See Tsushio et al, Hydrogenation Properties of Mg-based Laves Phase Alloys, Journal of Alloys and Compounds, 269 (1998), 219-223. Tsushi et al. determined that no hydrides of these alloys have been reported, and they did not succeed in modifying MgNi.sub.2 alloys to form hydrogen storage materials.
Finally some work has been done on high Mg content alloys or elementally modified Mg. For instance, in co-pending U.S. application Ser. No. 09/066,185, now U.S. Pat. No. 5,976,216, Sapru, et al have produced mechanically alloyed Mg-Ni-Mo materials containing greater than about 80 atomic percent Mg, for thermal storage of hydrogen. These alloys are formed by mixing the elemental ingredients in the proper proportions in a ball mill or attritor and mechanically alloying the materials for a number of hours to provide the mechanical alloy. While these alloys have improved storage capacities as compared with Mg.sub.2 Ni alloys, they have low plateau pressures.
Another example of modified high Mg content alloy is disclosed in U.S. Pat. No. 4,431,561 ('561) to Ovshinsky et al., the disclosure of which is hereby incorporated by reference. In the '561 patent, thin films of high Mg content hydrogen storage alloys were produced by sputtering. Once again, while these materials had high storage capacities, their plateau pressures were low, thus not providing adequate hydrogenation properties.
In U.S. Pat. No. 4,623,597 ("the '597 patent"), the contents of which are incorporated by reference, one of the present inventors, Ovshinsky, described disordered multicomponent hydrogen storage materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, intermediate range order, and/or polycrystalline (lacking long range compositional order) wherein the polycrystaline material includes topological, compositional, translational, and positional modification and disorder. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the '597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed the batteries to be utilized most advantageously as secondary batteries, but also as primary batteries.
Tailoring of the local structural and chemical order of the materials of the '597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the anodes of the '597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. The disordered material had the desired electronic configurations which resulted in a large number of active sites. The nature and number of storage sites was designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provided a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods therebetween resulting in long cycle and shelf life.
The improved battery of the '597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the '597 patent, be chemically modified with other elements to create a greatly increased density of catalytically active sites for hydrogen dissociation and also of hydrogen storage sites.
The disordered materials of the '597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The internal topology that was generated by these configurations also allowed for selective diffusion of atoms and ions. The invention that was described in the '597 patent made these materials ideal for the specified use since one could independently control the type and number of catalytically active and storage sites. All of the aforementioned properties made not only an important quantitative difference, but qualitatively changed the materials so that unique new materials ensued.
The disorder described in the '597 patent can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced into the host matrix by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, or by introducing regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
These same principles can be applied within a single structural phase. For example, compositional disorder is introduced into the material which can radically alter the material in a planned manner to achieve important improved and unique results, using the Ovshinsky principles of disorder on an atomic or microscopic scale.
One advantage of the disordered materials of the '597 patent were their resistance to poisoning. Another advantage was their ability to be modified in a substantially continuous range of varying percentages of modifier elements. This ability allows the host matrix to be manipulated by modifiers to tailor-make or engineer hydrogen storage materials with all the desirable characteristics, i.e., high charging/discharging efficiency, high degree of reversibility, high electrical efficiency, long cycle life, high density energy storage, no poisoning and minimal structural change.
These same attributes have not, until now, been achieved for thermal hydrogen storage alloys. Therefore, there has been a strong felt need in the art for high capacity, high plateau pressure/low heat of formation, low cost, light weight thermal hydrogen storage alloy materials.